Histology Fact Sheet: Heart

Layers of the Heart

The heart consists of three layers: epicardium, myocardium and endocardium.

Histology of the Epicardium

The epicardium is the outer layer of the heart. When examining the histology of the epicardium, it is apparent that it contains the blood vessels and nerves which supply the heart. The epicardium contains abundant adipose tissue.

Histology of the Myocardium

The myocardium is the middle layer of the heart. The myocardium is the muscular layer of the heart. It is thickest in the left ventricle, since the left ventricle is responsible for pumping blood throughout the systemic circulation.  Upon examining the histology of the myocardium, it is apparent that the myocardium is composed of cardiac muscle.

Histology of the Endocardium

The endocardium is the inner layer of the heart. The endocardium contains blood vessels.  The endocardium itself is layered.

Endothelium

The innermost portion of the endocardium is composed of endothelium, a simple squamous epithelium.

Smooth Muscle and Connective Tissue

The middle layer of the endocardium is connective tissue and smooth muscle.

Subendocardial Layer

The outer layer of the endocardium is the subendocardial layer.

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Histology of Cardiac Muscle

On a histology slide, branching cells are seen in cardiac muscle. Histology hint from Sarah Bellham: this is an important point, as both skeletal muscle and cardiac muscle are striated. The branching seen with cardiac muscle is one of the things that can be used to distinguish between cardiac muscle and skeletal muscle on a histology slide.

On a histology slide of cardiac muscle, the nucleus is located centrally. Histology hint from Sarah Bellham: This is an important point, as both skeletal muscle and cardiac muscle are striated. The centrally placed nucleus seen in cardiac muscle is one of the things that can be used to distinguish between cardiac muscle and skeletal muscle on a histology slide.

Occasionally, cardiac muscle is bi-nucleated.

On a histology slide of cardiac muscle, intercalated discs are seen. Intercalated discs are specialized junctions between cardiac cells.

Endomysium is the connective tissue covering of an individual muscle cell.

Conduction System

A cardiac impulse is transmitted through a specific pathway of modified cardiac tissue within the heart. The pacemaker of the heart is the sinoatrial node (SA node). The impulse then goes to the AV node. It goes through the ventricles via the bundle of His. The bundle of His is also called the atrioventricular bundle. This divides into right and left bundle branches. From there the impulse is transmitted into Purkinje fibers.

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Sources:

Histology: A Text and Atlas
Michael H. Ross/Edward J. Reith

Human Anatomy
Elaine N. Marieb; Jon Mallatt